1. Field of the Invention
The invention is directed to pressure measurement devices and, more specifically, to novel deadweight pressure standards which utilize non-hysteresis sylphons and one or more non-cylindrical pistons.
2. Description of Related Art
Mechanical pressure sensors have been known for centuries. Although traditional U-tube manometers have been superceded by more sophisticated devices, all pressure sensors require scheduled, periodic maintenance and recalibration. Pressure sensors can be recalibrated in the field or at a calibration laboratory. From the standpoint of accuracy, laboratory calibration is preferred, but often is not possible or necessary. In the laboratory, calibration devices from one or more of the following categories may be employed. The first category includes deadweight testers and U-tube manometers that provide primary, baseline standards. The second category includes laboratory or field standard calibration devices that are periodically recalibrated against the primary standard. Secondary standards are less accurate than the primary standard, but provide a more convenient means for testing and adjusting other instruments.
With reference to FIG. 1, a simplified prior art deadweight tester includes a dead weight 101 supported above a primary piston 102. The force of gravity on dead weight 101 causes primary piston 102 to exert a downward force on a fluid in a fluid reservoir 103. A secondary piston 104 is pressed into fluid reservoir 103 by means of a screw 105. Fluid reservoir 103 is coupled to a pressure gauge 106 or transducer under test. Primary piston 102 has a cross-sectional area A, and the weight W of dead weight 101 plus primary piston 102 is selected to correspond to a desired calibration pressure P using the equation P=(W/A). Secondary piston 104 pressurizes the fluid in fluid reservoir 103 by pressing more fluid into the reservoir until dead weight 101 lifts off its support.
In the United States, the National Institute of Standards and Technology (NIST) provides certified weights and calibrates laboratory piston gauges by measuring the diameter of primary piston 102. Deadweight testers can be used to calibrate at pressure levels as low as 5 psig (35 KPa) and as high as 100,000 psig (690 MPa). NIST calibrated deadweight testers can be accurate to 5 parts in 100,000 at pressures below 40,000 psig (280 MPa). For an industrial quality deadweight tester, error is typically 0.1% of span.
Several techniques have been proposed for improving the accuracy of deadweight pressure testers. Some designs adopt sophisticated temperature compensation schemes, whereas other designs utilize mechanisms for rotating the primary piston in its cylinder along with gas lubrication to negate the effect of friction. Further accuracy enhancements are possible when non-cylindrical pistons are used. An illustrative prior art deadweight pressure tester employing a noncylindrical piston is shown in FIG. 2. This pressure tester is also described in Russian Patent No. 1,008,633.
The prior art deadweight pressure tester of FIG. 2 includes a non-cylindrical piston 202 inside a nozzle 201. Non-cylindrical piston 202 may, but need not, have a substantially spherical configuration. Non-cylindrical piston 202 is tied to suspension 203 and weights 204. An air flow chamber 205 through pipes 206 and 207 is connected to a throttle 208 and volume 209. The throttle 208 is connected to an air flow chamber 211 of an air flow regulator 212 through a pipe 210. A volume 209 is connected to a dead end chamber 214 through a pipe 213.
A first membrane 215 and a second membrane 216 each include non-flexible disks that are fixed, or fastened, or tied, to a first valve 217 through a first rod 218. First valve 217 controls air flow through a first nozzle 271. The effective area of first membrane 215 is greater than the effective area of second membrane 216. A chamber 219 is open to the atmosphere. Air flow chamber 211 is connected to a dead end chamber 223 of a pressure regulator 225 through a pipe 221. An input chamber 220 is connected to an air flow chamber 224 of pressure regulator 225 through a pipe 222.
A third membrane 226 and a fourth membrane 227 each include non-flexible disks that are fixed, or fastened, or tied, to a second valve 228 through a second rod 229. Second valve 228 controls air flow through a second nozzle 282. The effective area of third membrane 226 is greater than the effective area of fourth membrane 227. A chamber 230 is open to the atmosphere. An input chamber 231 is connected to a pressure supply 233 through a pipe 232.
Conceptually, the prior art pressure tester of FIG. 2 transforms gravitational forces exerted on non-cylindrical piston 202, suspension 203 and weights 204 into a pressure P. When pressure supply 233 is applied at pipe 232, a pressure tester output pressure is generated within chamber 205. This output pressure balances the gravity force of non-cylindrical piston 202, suspension 203, and weights 204. When the mass of weights 204 is increased, non-cylindrical piston 202 is forced in a downward direction (towards the earth), and clearance between nozzle 201 and non-cylindrical piston 202 is decreasing. The pressure inside chamber 205 increases to an equilibrium value that is sufficient to balance the greater weight of weights 204.
In order to secure a desired level of high accuracy for the prior art pressure tester of FIG. 2 across a wide range of pressures, it is necessary to maintain the pressure within chamber 220 proportional to the pressure tester's output pressure. Accordingly, pressure regulator 225 provides the function of maintaining a pressure within chamber 220 that is proportional to a desired output pressure. For example, the mass of weights 204 is increased. The pressure within chamber 223 then breaks the equilibrium balance of pressure regulator 225. Second valve 228 moves and, as a result, the pressure within chamber 224 increases until a new equilibrium balance of regulator 225 is achieved. Fluctuations in supply pressure 233 do not affect the accuracy of the pressure tester shown in FIG. 2 because any change in supply pressure is automatically compensated by second valve 228. Volume 209 is employed to dynamically stabilize the operation of the pressure tester of FIG. 2.
Presently existing non-cylindrical deadweight pressure standards have limited stability and accuracy. These shortcomings are caused by dry friction between first valve 217 and first nozzle 271, dry friction between second valve 228 and second nozzle 282, instability of first, second, third and fourth membranes 215, 216, 226, 227 as a function of pressure or time, hysteresis of first, second, third, and fourth membranes 215, 216, 226, 227, and instability of throttle 208 of air flow regulator 212.